docs: def COV_motionless_mixer(Ki, Q1, Q2, pipe_diameters):
      r'''Calculates CoV of a motionless mixer with a regression parameter in
      [1]_ and originally in [2]_.
  
      .. math::
          \frac{CoV}{CoV_0} = K_i^{L/D}
  
      Parameters
      ----------
      Ki : float
          Correlation parameter specific to a mixer's design, [-]
      Q1 : float
          Volumetric flow rate of larger stream [m^3/s]
      Q2 : float
          Volumetric flow rate of smaller stream [m^3/s]
      pipe_diameters : float
          Number of diameters along tail pipe for CoV calculation, 0 to 5 []
  
      Returns
      -------
      CoV : float
          Standard deviation of dimensionless concentration [-]
  
      Notes
      -----
      Example 7-8.3.2 in [1]_, solved backwards.
  
      Examples
      --------
      >>> COV_motionless_mixer(Ki=.33, Q1=11.7, Q2=2.74, pipe_diameters=4.74/.762)
      0.0020900028665727685
  
      References
      ----------
      .. [1] Paul, Edward L, Victor A Atiemo-Obeng, and Suzanne M Kresta.
         Handbook of Industrial Mixing: Science and Practice.
         Hoboken, N.J.: Wiley-Interscience, 2004.
      .. [2] Streiff, F. A., S. Jaffer, and G. Schneider (1999). Design and
         application of motionless mixer technology, Proc. ISMIP3, Osaka,
         pp. 107-114.
      '''
      Cv = Q2/(Q1 + Q2)
      COV0 = ((1-Cv)/Cv)**0.5
      COVr = Ki**(pipe_diameters)
      COV = COV0*COVr
      return COV